1. Field of the Invention
The present invention relates to a method for forming a composite structure made of a brittle material such as a ceramic or a metalloid formed on a substrate surface. More particularly, the present invention relates to a composite structure forming method in which an aerosol generated by scattering brittle material fine particles in a gas is ejected and caused to collide with a substrate at high speed to form a structure made of the brittle material.
2. Description of the Prior Art
Generally, when a ceramic sintered body is formed, a liquid phase sintering is carried out in which a sintering assistant is added to make the inter-jointing of ceramic particles easier so as to form a liquid phase near the boundary face at which the particles join.
Hot pressing is a known method for forming a high-density sintered body without using the sintering assistant. A vapor deposition method, such as PVD and CVD, or a thermal spraying method are also known methods of forming a coat such as a metal or a ceramic on a substrate surface.
On the other hand, a gas deposition method (published in a metal magazine “KINZOKU” issued in January 1989 by Mr. KASHU, Seiichiro) and an electrostatic fine particle coating method (published in an advance printing used in an academic lecture meeting by Mr. Ikawa et al. in the Precision Machine Society of Japan held in the autumn of 1977) are also known as new coat-forming methods. In the former, it is a basic principle that ultra-fine particles such as metal or ceramic are made into an aerosol by gas agitation and accelerated through a minute nozzle. When the ultra-fine particles collide with a substrate, a part of their kinetic energy is converted to thermal energy to cause sintering between the fine particles or between the fine particles and the substrate. In the latter, it is a basic principle that fine particles are charged and accelerated using a gradient of an electric field, and then sintered in the same manner as in the gas deposition method using the thermal energy generated when the fine particles collide with the substrate.
Further, examples of the prior art which have improved the gas deposition method or the electrostatic fine particle coating method stated above are disclosed in Japanese Unexamined Patent Publication No. HEI 8-81774, Japanese Unexamined Patent Publication No. HEI 10-202171, Japanese Unexamined Patent Publication No. HEI 11-21677 or Japanese Unexamined Patent Publication No. 2000-212766.
In the art disclosed in Japanese Unexamined Patent Publication No. HEI 8-81774, two kinds of metal or organic substances with different melting points are heated to evaporation by resistance wire heating, electron beam heating, high-frequency induction heating, sputtering, arc plasma or the like to produce ultra-fine particles of 0.1 μm or less of which the surface is very active. These ultra-fine particles are sprayed, every metal with a different melting point, on a substrate using a nozzle based on sectional CAD data for a three-dimensional shape. This operation is repeated to form a substance with a three-dimensional shape consisting of two kinds of metals with different melting points. The substance with a three-dimensional shape is then heated at an intermediate temperature between the melting points of the two kinds of metal to melt and remove a metal portion with a low melting point, thereby leaving a metal portion with a high melting point.
In the art disclosed in Japanese Unexamined Patent Publication No. HEI 10-212171, the ultra-fine particles obtained by heating and evaporating the metal or the organic substance using resistance wire heating, electron beam heating, high-frequency induction heating, sputtering, arc plasma or the like as stated above are sprayed on the substrate through an opening of a mask. In this manner, a substance of a three-dimensional shape with no sagging shoulders is formed.
In the art disclosed in Japanese Unexamined Patent Publication No. HEI 11-21677, when an aerosol including the above-mentioned ultra-fine particles is conveyed or the metal or a ceramic is heated and evaporated, a classifying device is provided in an intermediate channel to prevent the ultra-fine particles from cohering together to become larger particles.
Referring to the art disclosed in Japanese Unexamined Patent Publication No. 2000-212766, when an ion beam, an atomic beam, a molecular beam or a low temperature plasma is irradiated on the ultra-fine particles of 10 nm˜5 μm (which are not obtained by heating and evaporation unlike the above-mentioned prior art), the ultra-fine particles are activated without melting. In such a condition, the activated ultra-fine particles are sprayed onto the substrate at a speed of ˜3 m/sec.˜300 m/sec. to promote inter-jointing of the ultra-fine particles thereby forming a structure.
In the liquid phase sintering using a general sintering assistant, a glassy phase including the sintering assistant is formed near a grain boundary. As a result, purity of the ceramics obtained does not increase, and it is difficult to form a compact body.
On the other hand, it is possible to form ceramics of high purity and compactness thanks to the atomization of ceramic particles, adoption of a high sintering temperature, baking under a pressurized environment such as the hot pressing method or the like, removal of the sintering assistant, etc. However, inclusive of the above, to effect baking is to let the particles join together by the diffusion of atoms and even though the raw powder is minute particles, particle growth is produced during heating. It is therefore impossible to let a formed subject remain as minute crystals. Namely, in baking, it is difficult to form a polycrystalline substance consisting of crystal grains of a nanometer level.
Further, during baking using a sintering assistant, a specific element segregates on a boundary face between the particles, resulting in preventing the accomplishment of the desired characteristics.
On the other hand, in PVD or CVD, there is a technical characteristic whereby a structure is formed by accumulation of atoms. Since a crystal plane of which the crystal growth energy is low grows faster, there is a characteristic structure that the crystal is oriented or the crystal is formed in a columnar shape from the substrate. It is therefore difficult to form a granular polycrystalline substance with disordered crystal orientation.
Referring to thermal spraying, compactness of the formed subject is attained thanks to atomization of the raw powder, processing at high temperature, environment under reduced pressure or the like. However, there is a technical characteristic whereby a surface layer of the raw powder is melted to collide with the substrate and let the particles join together. Accordingly, there is a problem that the crystals of the formed subject are shaped by deposition of flat particles in layers or non-molten particles are mixed into the formed subject. It is also difficult to form the polycrystalline substance consisting of crystal grains in the nanometer level. From the process point of view, there is still a problem whereby all the techniques above require a high temperature environment from several hundreds to 10,000° C. and the energy input is quite large.
Referring further to formation of a ceramic coat by a sol-gel method, a technique has been developed that can form a coat of which the crystallite is comparatively small at a low temperature. However, the coat thickness attained in one coat forming process is generally at a level from several nm to several hundreds nm, and when a thick coat is formed, it is necessary to repeat this process. In this case, it is substantially necessary to apply a heat treatment to strengthen the coat that is already applied, wherein particle growth is caused in such a coat layer. There is a problem that the compactness does not increase when a coat is formed at a low temperature at which the particle growth is not produced. A problem whereby a crack is produced on the coat when the coat forming process is repeated many times has not yet been solved. Further, the ceramics coat forming method for a fine structure such as the sol-gel method or a deposition method in a solution is a wet process in many cases. Thus, there is some possibility that other solutes or solvents in the solution are mixed in the coat to generate deterioration of the coat characteristics or deformation of composition.
In the methods disclosed in Japanese Unexamined Patent Publication No. HEI 8-81774, Japanese Unexamined Patent Publication No. HEI 10-202171, and Japanese Unexamined Patent Publication No. HEI 11-21677, heating means for obtaining the ultra-fine particles (such as resistance wire heating, electron beam heating, high-frequency induction heating, sputtering or arc plasma) is needed. Further, since the basic principle is that the kinetic energy is converted to thermal energy upon collision to effect sintering, a particle size of the structure formed on the substrate is larger than that of the ultra-fine particles of the raw material due to particle growth.
On the other hand, applicants of the present invention have improved the techniques disclosed in Japanese Unexamined Patent Publication No. 2000-212766. As a result, it became clear that a brittle material such as a ceramic or a metalloid shows a different behavior from a metal (spreading material).
In the brittle material, it was possible to form a structure without irradiating with an ion beam, atomic beam, molecular beam, low temperature plasma, etc., in other words, without using any particular activation means. However, even though fine particles of 10 nm˜5 mm and a collision speed of 3 m/sec.˜300 m/sec. which are the conditions described in the published specification, are met, there are new problems in that peel strength of the structure is not enough, partial peeling can easily occur, or the density is not uniform.